I took a look around for the Kindle store for foreign books as well and couldn't find any.  Instead, when I specifically put in 日本語 as a search term, I got some really bizarre results:

http://www.amazon.com/Kindle-eBooks/b/ref=amb_link_355831402_37?ie=UTF8&node=154606011&pf_rd_m=ATVPDKIKX0DER&pf_rd_s=left-1&pf_rd_r=146CFJJ242HJZQ2P6E5S&pf_rd_t=101&pf_rd_p=1337674602&pf_rd_i=1286228011#/ref=sr_pg_2?rh=n%3A133140011%2Cn%3A!133141011%2Cn%3A154606011%2Ck%3A%E6%97%A5%E6%9C%AC%E8%AA%9E&page=2&keywords=%E6%97%A5%E6%9C%AC%E8%AA%9E&ie=UTF8

Oh, wait, here is a different search which is a "bit" better:

http://www.amazon.com/s/ref=sr_nr_n_5?rh=n%3A283155%2Cn%3A!44258011%2Cn%3A!251254011%2Cn%3A3118571%2Cp_n_feature_browse-bin%3A618073011%2Cn%3A3151591&bbn=3118571&ie=UTF8&qid=1323219998&rnid=3118571

But it is unlikely your BMAJW would be satisfied with that.

There is a huge dearth of ePubs available when you are in a foreign country.  The iBooks app for purchasing English novels on the iPad while in Japan is borderline pointless.  Unless it is published by Apple themselves, you pretty much only get access to Project Gutenberg (oh, and Winnie the Pooh which I liked but would also like the sequel House at Pooh Corner.)  Fortunately, for English books it is relatively easy to...um...acquire shall we say a HUGE library of ePubs without trying very hard.

There are a number of accessories for Kindle on Amazon.jp but all the publications have to come from the US based store.  Likewise, I think there are a number of Japanese books for iBooks app in the Japanese iTunes store but (other than the exception listed above) not much in anything else.

You also might want to have a look around and see if the BMAJW would be able to properly a Kindle Fire.  I am not slighting her in any fashion.  A lot of articles I have read recently seem to suggest that the Fire is a bit of a nuisance to actually use (while not actually having attempted to myself.)